2,3,4,5-tetrahydrobenzo[f][1,4]thiazepines are important compounds because of their biological activities, as disclosed, for example, in U.S. Pat. Nos. 5,416,066 and 5,580,866 and published US Patent Applications Nos. 2005/0215540, 2007/0049572 and 2007/0173482.
Synthetic procedures exist for the preparation of 2-oxo-, 3-oxo-, 5-oxo- and 3,5-dioxo-1,4-benzothiazepines and for 2,3-dihydro-1,4-benzothiazepines. However, relatively few publications describe the preparation of 2,3,4,5-tetrahydrobenzo-1,4-thiazepines that contain no carbonyl groups, and most of these involve reduction of a carbonyl group or an imine. Many of the routes described in the literature proceed from an ortho-substituted arene and use the ortho substituents as “anchors” for the attachment of the seven-membered ring. Essentially all the preparatively useful syntheses in the literature that do not begin with an ortho-substituted arene employ a modification of the Bischler-Napieralski reaction in which the carbon of the acyl group on the γ-amide becomes the carbon adjacent the bridgehead and the acyl substituent becomes the 5-substituent. Like earlier mentioned syntheses, the Bischler-Napieralski synthesis requires reduction of an iminium intermediate.
It would be useful to have an intramolecular reaction for the direct construction of 2,3,4,5-tetrahydrobenzo[1,4]thiazepines that would allow more flexibility in the 4- and 5-substituents and that would avoid a separate reduction step. The Pictet Spengler reaction, in which a β-arylethylamine such as tryptamine undergoes 6-membered ring closure after condensation (cyclization) with an aldehyde, has been widely used in the synthesis of 6-membered ring systems over the past century and might be contemplated for this purpose. The Pictet Spengler reaction, however, has not been generally useful for 7-membered ring systems such as 1,4-benzothiazepines. A plausible explanation is that the failure of the reaction for typical arenes was due to the unfavorable conformation of the 7-membered ring. There are two isolated examples of an intramolecular Pictet-Spengler-type reaction producing a good yield of a benzothiazepine from the addition of formaldehyde. In one case, the starting material was a highly unusual activated arene (a catechol derivative) [Manini et al. J. Org. Chem. (2000), 65, 4269-4273]. In the other case, the starting material is a bis(benzotriazolylmethyl)amine that cyclizes to a mono(benzotriazolyl)benzothiazole [Katritzky et al. J. Chem. Soc. P1 (2002), 592-598].